The Irish Citizen Army (ICA) formed during the 1913 Lockout,[5] was informally the military wing of the Labour Movement. The ICA took part in the 1916 Rising.[6] The ICA was revived during Peadar O'Donnell's Republican Congress but after the 1935 split in the Congress most ICA members joined the Labour Party.

The British Labour Party had previously organised in Ireland, but in 1913 the Labour NEC agreed that the Irish Labour Party would have organising rights over the entirety of Ireland. A group of trade unionists in Belfast objected and the Belfast Labour Party, which later became the nucleus of the Northern Ireland Labour Party, remained outside the new Irish party.